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ThS. Trần Thị Thanh Nga Khoa CNTT, Trường ĐH Nông Lâm TPHCM Email: ngattt@hcmuaf.edu.vn Java Basic - Arrays 1

Array Basics  An array is used to store a collection of data  An array as a collection of variables of the same type .  Instead of declaring individual variables, such as number0 , number1 , and number99 ,  you declare one array variable such as numbers and use numbers[0] , numbers[1] , and numbers[99] to represent individual variables. Java Basic - Arrays 2

Declaring Array Variables  Syntax: elementType [] arrayRefVar;  Example: double [] myList; Java Basic - Arrays 3

Creating Arrays  The declaration of an array variable does not allocate any space in memory for the array.  It creates only a storage location for the reference to an array.  If a variable does not contain a reference to an array, the value of the variable is null .  You cannot assign elements to an array unless it has already been created. Java Basic - Arrays 4

Creating Arrays  Creating an array by using the new operator.  Syntax: arrayRefVar = new elementType[arraySize];  This statement does 2 things:  (1) it creates an array using new elementType[array-Size];  (2) it assigns the reference of the newly created array to the variable arrayRefVar . Java Basic - Arrays 5

Creating Arrays  Declaring an array variable, creating an array, and assigning the reference of the array to the variable can be combined: elementType [] arrayRefVar = new elementType[arraySize]; or: elementType arrayRefVar[] = new elementType[arraySize];  Example: double [] myList = new double [10];  To assign values to the elements, use the syntax: arrayRefVar[index] = value; Java Basic - Arrays 6

Assigning Arrays  To assign values to the elements, use the syntax: arrayRefVar[index] = value; myList[0] = 5.6; myList[5] = 34.33; myList[1] = 4.5; myList[6] = 34.0; myList[2] = 3.3; myList[7] = 45.45; myList[3] = 13.2; myList[8] = 99.993; myList[4] = 4.0; myList[9] = 11123; Java Basic - Arrays 7

Example of Array Java Basic - Arrays 8

Array Size and Default Values  When space for an array is allocated, the array size must be given, specifying the number of elements that can be stored in it.  The size of an array cannot be changed after the array is created.  Size can be obtained using myList.length . Java Basic - Arrays 9

Array Indexed Variables  The array elements are accessed through the index.  They range from 0 to arrayRefVar.length-1 .  Each element in the array is represented using the following syntax, known as an indexed variable: arrayRefVar[index];  An indexed variable can be used in the same way as a regular variable: myList[2] = myList[0] + myList[1]; Java Basic - Arrays 10

Array Initializers  Java has a shorthand notation, known as the array initializer , which combines in one statement declaring an array, creating an array, and initializing . elementType[] arrayRefVar = {value 0 , value 1 , ..., value k };  Example:  double [] myList = { 1.9 , 2.9 , 3.4 , 3.5 };  Equivalent to the statements shown below: double [] myList = new double [ 4 ]; myList[ 0 ] = 1.9; myList[ 1 ] = 2.9; myList[ 2 ] = 3.4; myList[ 3 ] = 3.5; Java Basic - Arrays 11

Processing Arrays  When processing array elements, you will often use a for loop — for two reasons:  All of the elements in an array are of the same type . They are evenly processed in the same fashion repeatedly using a loop.  The size of the array is known, it is natural to use a for loop. Java Basic - Arrays 12

Example: Initializing arrays with input values double[] myList = new double[10]; Scanner input = new Scanner(System. in); System. out.print("Enter " + myList.length + “ values: "); for (int i = 0; i < myList.length; i++) myList[i] = input.nextDouble(); Java Basic - Arrays 13

Example: Initializing arrays with random values double[] myList = new double[10]; for (int i = 0; i < myList.length; i++) { myList[i] = Math. random() * 100; } Java Basic - Arrays 14

Example: Displaying arrays for (int i = 0; i < myList.length; i++) { System. out.print(myList[i] + “ \t"); } Java Basic - Arrays 15

Example: Summing all elements double[] myList = new double[10]; //gán giá trị cho từng phần tử mảng //… double total = 0; for (int i = 0; i < myList.length; i++) { total += myList[i]; } Java Basic - Arrays 16

Example: Finding the largest element double[] myList = new double[10]; double max = myList[0]; for (int i = 1; i < myList.length; i++) { if (myList[i] > max) max = myList[i]; } Java Basic - Arrays 17

Example: Finding the smallest index of the largest element double[] myList = new double[10]; double max = myList[0]; int indexOfMax = 0; for (int i = 1; i < myList.length; i++) { if (myList[i] > max) { max = myList[i]; indexOfMax = i; } } Java Basic - Arrays 18

Example: Random shuffling double[] myList = new double[10]; for (int i = 0; i < myList.length; i++) { // Generate an index j randomly int index = (int) (Math. random() * myList.length); // Swap myList[i] with myList[j] double temp = myList[i]; myList[i] = myList[index]; myList[index] = temp; } Java Basic - Arrays 19

Example: Shifting elements double[] myList = new double[10]; double temp = myList[0]; //Retain the first element // Shift elements left for (int i = 1; i < myList.length; i++) { myList[i - 1] = myList[i]; } // Move the first element to fill in the last position myList[myList.length - 1] = temp; Java Basic - Arrays 20

For-each Loops  You can traverse the array sequentially without using an index variable. for ( double u : myList ) { System.out.println(u); }  “for each element u in myList do the following.”  the variable, u , must be declared the same type as the elements in myList Java Basic - Arrays 21

Copying Arrays  You need to duplicate an array or a part of an array.  You could attempt to use the assignment statement (=), as follows: list2 = list1; Java Basic - Arrays 22

Copying Arrays  Assigning one array variable to another array variable actually copies one reference to another and makes both variables point to the same memory location.  There are three ways to copy arrays:  Use a loop to copy individual elements one by one.  Use the static arraycopy method in the System class.  Use the clone method to copy arrays. Java Basic - Arrays 23

Copying Arrays int[] list1 = {1, 2}; int[] list2 = list1.clone(); list1[0] = 7; list1[1] = 8; System. out.println("list1 is " + list1[0] + ", " + list1[1]); System. out.println("list2 is " + list2[0] + ", " + list2[1]); Java Basic - Arrays 24

Passing Arrays to Methods public static void printArray(int[] array) { for (int i = 0; i < array.length; i++) { System. out.print(array[i] + " "); } } You can invoke it by passing an array: printArray(new int[]{3, 1, 2, 6, 4, 2}); Java Basic - Arrays 25

Pass-by-value  Java uses pass-by-value to pass arguments to a method.  There are important differences between passing the values of variables of primitive data types and passing arrays .  For an argument of a primitive type, the argument’s value is passed.  For an argument of an array type: the value of the argument is a reference to an array; this reference value is passed to the method.  Semantically, it can be best described as pass-by-sharing. Java Basic - Arrays 26

Pass-by-value public class Test { public static void main(String[] args) { int x = 1; // x represents an int value int[] y = new int[10]; // y represents an array of int values methodE(x, y); // Invoke m with arguments x and y System. out.println("x is " + x); System. out.println("y[0] is " + y[0]); } public static void methodE(int number, int[] numbers) { number = 1001; // Assign a new value to number numbers[0] = 5555; // Assign a new value to numbers[0] } } Java Basic - Arrays 27

Pass-by-value The primitive type value in x is passed to number , and the reference value in y is passed to numbers . Java Basic - Arrays 28

public class TestPassArray { public static void main(String[] args) { int[] a = { 1, 2 }; // Swap elements using the swap method System. out.println("Before invoking swap"); System. out.println("array is {" + a[0] + ", " + a[1] + "}"); swap(a[0], a[1]); System. out.println("After invoking swap"); System. out.println("array is {" + a[0] + ", " + a[1] + "}"); // Swap elements using the swapFirstTwoInArray method System. out.println("Before invoking swapFirstTwoInArray"); System. out.println("array is {" + a[0] + ", " + a[1] + "}"); System. out.println("After invoking swapFirstTwoInArray"); System. out.println("array is {" + a[0] + ", " + a[1] + "}"); } Java Basic - Arrays 29

/** Swap two variables */ public static void swap(int n1, int n2) { int temp = n1; n1 = n2; n2 = temp; } /** Swap the first two elements in the array */ public static void swapFirstTwoInArray(int[] array) { int temp = array[0]; array[0] = array[1]; array[1] = temp; } } Java Basic - Arrays 30

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Example Java Basic - Arrays 32